Flexibility in Signatures

Rust has several ways to write code that works with more than one type. Each is a different design commitment:

Mechanism	Dispatch	Set of types	Reads as
`enum`	`match`	Closed: you list the variants	“These, and only these.”
Generics `<T: Trait>`	Static	Open, fixed at compile time	“Anything with this capability, known now.”
`dyn Trait`	Dynamic	Open, chosen at runtime	“Anything with this capability, chosen later.”
Composition	None	Not polymorphism	“This is made of parts.”

A decision procedure, usable in review and in prompts:

Do you control the full set of variants, and is it small and stable? Use an enum. As long as matches avoid _ arms, adding a variant produces a compile error everywhere it’s not handled.
Do downstream users need to plug in their own types? Use a trait. Prefer generics; use dyn for heterogeneous collections or to keep type parameters out of a public signature.
Is the relationship just “has-a”? Use composition.

In trait definitions

A generic parameter on the trait (trait Store<V>) allows one type to implement the trait several times, once per V.
An associated type (trait Store { type V; }) allows one implementation with one choice of V. This is usually what you want.